Secure documents have been produced in the past by letterpress, offset, and intaglio printing processes, including such processes adapted for embedding hidden information or security images. The hidden information or security images, often referred to as latent images, are printed so as to be effectively invisible to the unaided eye but can be detected using various optical enhancements. Document embedded security features deter counterfeiting of valuable papers, important records, ID cards, and financial instruments such as checks, currency, so that original documents can be verified and unauthorized copies of these documents can be readily distinguished from the originals.
The latent images, which often contain covert information or other indicia such as security codes, graphics, or information within the original documents, can be hidden within visually integrated settings so as to be practically invisible to the unaided eye under normal viewing conditions. Such settings for hiding the latent images range from simple background tints to the remainders of composite images within which the latent images are hidden. However, the latent images are otherwise distinguishable through the use of a visual aid, such as a magnifier or reader. Upon reproduction, such as by scanning or copying of the originals, the latent images can be arranged either to disappear or to become more pronounced. That is, either the absence of the latent image or the visible presence of the latent image in a copy can be construed as an indication of a non-original document.
Such latent images can be produced by embedding one line-screen pattern within another line-screen pattern. Each of the line-screen patterns can be formed by an array of print elements, such as dots, dashes, line segments, or other elemental shapes, in an arrangement of systematically spaced lines. The two line-screen patterns can be formed or otherwise camouflaged so as to be indistinguishable from one another to the unaided eye, but can differ in some respect that can be optically exploited for viewing the latent image. For example, the two line-screen patterns can be printed in the same color and at the same print density but can differ in orientation or line frequency.
A first line-screen pattern can be printed in defined areas having one or more shapes for forming the latent image. A second line-screen pattern can be printed throughout an adjoining or an enlarged area forming a visually integrated setting, such as a complementary background or a remaining portion of a composite image. Each of the two line patterns can be formed by a plurality of line patterns in different colors so long as the multiple line patterns that form the latent image match the color and overall appearance of the one or more line patterns that form the visually integrated setting.
Although invisible to the unaided eye, the latent images can be detected by various optical enhancements such as through optical overlays that increase contrast between the latent images and their visually integrated settings. For example, viewers containing screen patterns that match one of the underlying screen patterns of the latent images or their visually integrated settings can fill in or obscure one line pattern with respect to another.
In addition to the authentication and validation of originals, the latent images can also be used to distinguish copies from originals. For example, the latent images themselves can be rendered visible by copying for revealing messages such as “void” or “unauthorized copy” on the reproductions. Alternatively, the latent images can be arranged so as not to reproduce, so the absence of the latent image indicates a copy.
Other developments for purposes of providing document protection are disclosed in the patent literature, as for example, in U.S. Pat. No. 5,018,767 issued May 28, 1991; U.S. Pat. No. 5,193,853 issued Mar. 16, 1993; and U.S. Pat. No. 3,675,948 issued Jul. 11, 1972; and U.S. Pat. No. 4,143,967 issued Mar. 13, 1979, all to Ralph C. Wicker. All of these patents, which are hereby incorporated by reference, disclose various means for providing methods and products to enable copies of documents to be distinguished from the originals, as for example, by a “large dot-small dot pattern”, a “close line-spaced pattern”, and images or indicia which are screen printed at minutely varied spaces and/or angles on the originals and are intended to produce a highly visible moiré pattern effect on the unauthorized copies. In this specification, the words “print”, “printed” and “printing” are used to refer to the making of an original document by any of a number of known printing means, including transferring images from one source to another, typically a paper medium, using a transfer agent such as ink or toner. The words “copy” and “copying” are used to refer to making copies from an original printed document also by any number of known copying means including by such means as digitally copying or scanning for output to a printer.
Copier and computer scanner-printer technologies continue to evolve in ways that can thwart intentions for rendering the latent images irreproducible in their original form. A goal of these technologies is to obtain copies as good as an original. Copiers, scanners, and other duplicating equipment, including desktop publishing systems, have become increasingly sophisticated in color reproduction, including the color matching of copies to color standards such as the Pantone Color Matching System.
Many of these document protection methods and products were developed before such significant improvements in copier and computer reproduction technologies and are no longer as effective for distinguishing copies from originals. Thus, further improvements in the area of document protection are needed, especially for distinguishing copies from originals to prevent unauthorized copying of valuable documents.